Greg,
I scaled mine shortly after I started running - having limited experience with that specific vehicle, and with hauling freight of any significant weight generally, - and because of how the vehicle handled when loaded, I wanted to know exactly what the numbers were. This occurred in early April and the weather was still kinda bad with alot of wind and rain, with occassional snow and below freezing temps. I was not 100% comfortable with my knowledge of the vehicle and it's capabilities at that point - and I'm still not ..... which I don't necessarily think is a bad thing - since it tends to make me more cautious.
Anyways, I scaled on my first load of significant weight - the BOL claimed a gross weight for the load of around 2000 lbs. - and I suspected that the shipper might be understating the weight of the cargo - so I scaled, loaded, at a Pilot on I-71 on the way down to the consignee. The vehicle scaled at 7560 lbs. with the fuel tank showing 1/3 full. After I dropped the load I scaled again and it was at 5780 lbs - and that may have been with a full tank of fuel .... don't recall offhand.
Like I said the reason I did this was I was not comfortable (nor happy) with the way the vehicle was handling when loaded. I ultimately decided to do something about the vehicle's suspension, addressing the parts of it that I felt were inadequate. You shouldn't necessarily assume that those modifications were done to run overweight - it might just be that I'd prefer to be safer when running legal weights.
One point on your numbers - all published unladen weight specs I have seen for the Sprinter INCLUDE a fuel tank of fuel and the weight of a driver (who knows where or how they come up with that figure ?) - so you are probably a little light on the capacity that you state - by the actual numbers mine is a little closer to 2800 lbs.
If you search back on EO a few years you will an article about the Sprinter that contains comments about the vehicle and it's capabilities - this was when the vehicle was relatively new and becoming popular. In that article you will find a few people talking about running very significant weights in a Sprinter - including a manager from a well-known expedite company based in MI talking about hauling 4K+ loads. There was another guy who said he had run a 4K or 5K load across the PA turnpike in his Sprinter - and would never do it again.
Not defending the practice - just pointing out that some do "get" it .... and some apparently don't.
What I found really amazing is that a company official would actually publically admit to this for a article ....... that they had to know would be published ....... go figure .....